Ethanol is mainly used as a vehicle fuel and a vehicle fuel additive, and at the meanwhile it is an important organic chemical raw material and is mainly used for the production of acetaldehyde, ethyl ether, acetic acid, ethyl acetate, ethylamine, or the like. The global production of ethanol in 2012 is up to 85.1 billion liters, while ethanol-gasoline in China has accounted for 20% or more of the total amount of gasoline consumption in China. During the twelfth five-year plan, the proportion for which the non-fossil energies in China account in the primary energies will increase to 11.4%, and at the meanwhile new fuel ethanol projects with crops as mainly raw materials will be no longer constructed. Therefore, the synthesis of ethanol from a plurality of pathways has important realistic sense and strategic sense for saving petroleum resources and reducing environmental pollution.
At present, the industrial methods for producing ethanol include fermentation method and ethylene hydration method (the ethanol, either prepared by fermentation method or ethylene hydration method, is typically an azeotrope of ethanol and water, and further dehydration is required to obtain absolute ethanol). The main raw materials of fermentation method are plants such as sugar cane, cassava, corn, etc. A large amount of corn in the United States is used to produce ethanol fuel, resulting in short supply and increased price of crops in the world. Therefore, a number of countries limit bioethanol projects to some extent. The ethylene hydration method uses phosphoric acid carried on silica gel or diatomaceous earth as a catalyst, and this process is firstly industrialized by Shell Corporation in 1947. Broadening sources of ethanol raw materials and reducing the dependence on petroleum resources have become hot spots of studies. It is considered to be one of the most promising schemes to directly prepare ethanol from a synthesis gas [Appl. Catal. A 261 (2004) 47, J. Catal. 261 (2009) 9]. Pan et al., [Nat. Mater. 6 (2007) 507] reports a Rh precious metal catalyst selectively carried on the inner wall of a carbon nanotube, which greatly increases the yield of ethanol directly prepared from synthesis gas. However, its further application is limited due to the use of the expensive precious metal Rh. Another scheme is to directly prepare ethanol from synthesis gas. On Nov. 1, 2011, a research and development project of a set of technique for preparing ethanol from at 30 thousand tons per year was started to be constructed by Jiangsu Suopu Group. This technique are developed by Dalian Institute of Chemical and Physics, Chinese Academy of Sciences, which indicates that this technique has been fully in the stage of industrial representative practice. After the project is established, it will be the first set of industrialized apparatus for preparing ethanol from coal through synthesis gas on an order of 10 thousand tons in the world. However, this scheme uses a catalyst of precious metal rhodium and the cost of the catalyst is relatively high. Furthermore, strictly, the product is mixed C1-C5 alcohols, and the selectivity of ethanol is less than 80% [Appl. Catal. A 407 (2011) 231, Appl. Catal. A 243 (2003) 155]. In addition, the homologation reaction for preparing ethanol from methanol is also intensively studied, and this scheme uses Co or Ru as a catalyst and an iodide as a promoter to perform reaction in a homogeneous system [J. Catal. 90 (1984)127, J. Mol. Catal. A 96 (1995) 215]. However, due to severe corrosion of apparatuses by iodides as well as complex reaction products and low selectivity of ethanol, the application of this method is limited to some extent.
In the newest scheme for directly synthesizing ethanol from synthesis gas, dimethyl ether is used as a raw material, methyl acetate is directly synthesized by carbonylation reaction, and ethanol is then prepared by hydrogenation. At present, this scheme is still in the research period, but it has remarkably large prospect for application. Recently, Tsubaki et al., [JP2008239539, ChemSusChem 3 (2010) 1192] achieves direct generation of ethanol from DME on H-MOR and Cu/ZnO catalysts, and it has been studied and found that the two catalysts have synergistic effect.
Methanol is also an important chemical raw material and a vehicle fuel additive, and is mainly used as a solvent and in the preparation of formaldehyde, acetic acid, and dimethyl ether and the process of MTG, MTO, etc. In 2011, the production of methanol in China is up to 20.35 million tons, and it is estimated that the production of methanol will further increase in the future as techniques such as MTO and the like are generalized. At present, the synthesis of methanol is achieved with a copper-based catalyst in a fixed bed reactor at 240-260° C. and 5-10 MPa.
At present, dimethyl ether can be prepared from synthesis gas with one step (using a bifunctional catalyst, methanol synthesis and methanol dehydration occur in one reactor) or can be synthesized by methanol dehydration. The synthesis gas may be prepared with non-petroleum energies, such as coal, biomass, natural gas, etc. If synthesizing ethanol at the same time of coproducing an amount of methanol can be achieved, methanol may not only be used as the final product, but also may be dehydrated to generate dimethyl ether. Dimethyl ether is carbonylated to generate methyl acetate, and methyl acetate is hydrogenated to generate the final product ethanol. The proportions of ethanol and methanol may be adjusted according to market demand to improve the flexibility of products and the operational motility of apparatuses, which has important realistic sense for developing new coal chemical industry. Therefore, a method for synthesizing ethanol and coproducing methanol under a co-feeding condition of synthesis gas and acetate is needed to be developed in the art.